
An airborne geophysical survey has confirmed the discovery of an impact crater under the town of Decorah in northeastern Iowa.
The crater's existence was first hypothesized in 2008 when geologists examining cuttings from water wells drilled near the town were surprised to find evidence of a previously unknown shale deposit. When geologist Robert McKay from the Iowa Geological Survey investigated further, he found something even more surprising: The shale deposit was nearly a perfect circle, roughly 5.5 kilometers across. Further analysis of sub-shale breccias by Bevan French, a geologist at the Smithsonian's National Museum of Natural History, revealed shocked quartz - a telltale sign of an impact. Together, the evidence added up to an exciting possibility: the existence of a previously unknown impact crater in the Midwest.
Earlier this year, more evidence accumulated when scientists at the U.S. Geological Survey (USGS) and the Iowa and Minnesota Geological Surveys conducted a high-resolution geophysical survey of the region to assess water resources and mineral resources. They were specifically mapping the Northeast Iowa Igneous Intrusive complex, which lies in the Midcontinent Rift System that formed about 1.1 billion years ago, and may contain valuable copper, nickel and platinum group metal resources.
Comment: A sonic boom from a military jet does not cause houses to shake...